It is known in the papermaking art to use drainage and retention aids. Such additives are introduced into the papermaking stock in order to facilitate drainage and/or to increase adsorption of fine particles and additives onto the cellulosic fibers so that they are retained with the fibers. Hereby the productivity in the papermaking process can be considerably increased and the use of drainage and retention aids thus offers substantial economic benefits.
Another important characteristic of the papermaking process is the formation of the paper sheet produced. Formation is determined by the variance in light transmission within a paper sheet, and a low variance indicates a good formation. The formation is affected by several factors, for example the manner in which the fibers are distributed, arranged and mixed within the paper sheet. Good formation is thus aimed at in the papermaking process in order to optimize the optical properties of the paper produced.
Small dosages of drainage and retention aids are generally beneficial to formation. However, even moderate dosages of drainage and retention aids may have an adverse effect on formation. As retention increases to a high level, the formation parameter may decline abruptly from good formation to poor formation. Poor formation gives rise to deteriorated paper quality and printability. Increased roughness of the paper surface is a further effect of poor formation which can have a negative impact on subsequent surface treatment such as coating. In addition, the problems of poor formation and hence deteriorated optical properties and printability may not be overcome by coating the paper since the result, normally, will not be as good as that obtained with paper produced under conditions resulting in good formation.
U.S. Pat. Nos. 4,980,025 and 5,368,833 and European Patent No. 656872 disclose the use of cationic acrylamide-based polymers and aluminium-containing silica sols as stock additives in papermaking. These systems are among the most efficient drainage and retention aids now in use.
According to the present invention it has been found that a combination of beneficial effects in terms of improved formation and very high drainage and retention performance can be obtained when aluminium-containing silica sols are used in conjunction with branched acrylamide-based polymers as stock additives in papermaking.